The combination of high powered, narrow linewidth laser optical sources and low-loss single-mode optical fiber, opens the possibility of signal degradation and increased bit error rates caused by a host of nonlinear fiber-related phenomena hitherto considered inconsequential with respect to conventional fiber-optic systems. These non-linear phenomena include stimulated Brillouin scattering ("SBS"), stimulated Raman scattering, self-phase modulation, and, if two or more optical channels are involved, cross-phase modulation and four-photon mixing.
Brillouin scattering within a fiber results from photons being scattered by localized refractive index variations induced by sound or acoustic waves. These refractive index variations are caused by sonic vibrations in the glass lattice that makes up the fiber core. Furthermore, owing to the dependence of refractive index on light intensity in the nonlinear regime, the presence of intense light in the fiber will also induce lattice vibrations which in-turn induce sound waves, that then scatter more light. Ultimately, light from an intense forward propagating signal (referred to as a "pump signal") can provide gain for a backward propagating or "Stokes" signal. This scenario is a classical description of SBS. SBS threshold power ("P.sub.SBS ") is arbitrarily defined as the level of input optical pump signal power ("P.sub.pump ") at which the power of the backward Stokes signal ("P.sub.Stokes ") becomes equal to P.sub.pump at the fiber input. SBS threshold power increases with the linewidth of the light being propagated along a fiber. For this reason, concern over the adverse effects of SBS was minimal until the debut of narrow linewidth laser sources. Now, however, as narrow linewidth lasers become more readily available, and as such lasers are likely to be the optical source of choice for future fiber transmission systems, SBS has the potential for significantly contributing to signal degradation at relatively low input power levels.
Some previous SBS suppression techniques required the use of exotic lasers, and/or employing particular types of specially fabricated fibers as the transmission media. This is obviously not a practical solution for use with existing optical fiber systems and commercially available laser technology. Other methods of increasing the power threshold at which SBS occurs involve employing lasers having increased linewidths to generate the signals propagated via the optical fiber systems. However, these methods have proved undesirable as an increased linewidth typically results in degrading the dispersion characteristics of the propagating optical signal.